Discovery and measurement of the Higgs boson in the WW decay channel

In the Standard Model of particle physics, the non-zero masses of the $W$ and $Z$ bosons and the fermions are generated through interactions with the Higgs field, excitations of which correspond to Higgs bosons. Thus, the experimental discovery of the Higgs boson is of prime importance to physics, and would confirm our understanding of fundamental mass generation. This thesis describes a search for the $gg \to H \to WW \to \ell\nu\ell\nu$ process of Higgs boson production and decay. It uses the LHC Run I dataset of $pp$ collisions recorded by the ATLAS detector, which corresponds to an integrated luminosity of $4.5~\text{fb}^{-1}$ at $\sqrt{s} = 7~\text{TeV}$ and $20.3~\text{fb}^{-1}$ at $\sqrt{s} = 8~\text{TeV}$. An excess of events is observed with a significance of 4.8 standard deviations, which is consistent with Higgs boson production. The significance is extended to 6.1 standard deviations when the vector boson fusion production process is included. The measured signal strength is $1.11^{+0.23}_{-0.21}$ at $m_{\text{H}} = 125~\text{GeV}$. A cross section measurement of $WW$ production, a major background to this search, is also presented using the $\sqrt{s} = 7~\text{TeV}$ dataset only.